new focus on ptfe - polymer extrusion & tubing …...teflon ptfe (or simply teflon ), has been...

Kevin J. Bigham, PhD. © 2018 Zeus Industrial Products, Inc. 1

NEW FOCUS ON PTFE

INTRODUCTION

Fluoropolymers exist in almost every aspect of our lives. From the non-stick coating

on our kitchen pots and pans to the packaging of our food, fluoropolymers have proven

to be a boon for industry while fulfilling critical needs. Generally chemically resistant

and resistant to burning, these materials share their most critical aspect: their carbon-

fluorine bonding. These exceptionally stable covalent bonds have been exploited in a

multitude of variations to produce the vast fluoropolymer industry as it is today.

Homopolymers – those made by combining single identical monomers, and

copolymers – those made from combining two different monomers, are but two

examples of the templates that have led to extensive fluoropolymer diversity (Fig. 1).

This vital fluoropolymer industry, however, sprang almost entirely from a single now-

classical fluoropolymer: polytetrafluoroethylene, or PTFE.

Figure 1: Landscape of popular fluoropolymers. Homopolymers like PTFE are produced

through the polymerization of identical monomer units. Copolymers incorporate two or more

monomers to produce the final polymer material.

DISCOVERY, SYNTHESIS, AND STRUCTURE

Polytetrafluoroethylene, or PTFE, was the result of a fortuitous albeit nonetheless

astute development by researchers at DuPont. Their work built upon earlier efforts

carried out by General Motors and DuPont together in the search for refrigerants [1].

Low-temperature liquid tetrafluoroethylene (TFE) had been left unattended in a

pressure vessel where later it was found to have spontaneously polymerized. The

resulting white waxy solid was discovered as a coating on the inside of the vessel walls;

this coating was PTFE. This finding occurred in 1938, but PTFE was not

Fluoropolymers

Homopolymers

PCTFECopolymers

PFA

PTFE

ETFEFEP ECTFETHV

(Terpolymer)

PVDF


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commercialized until 1946 due to its use during WWII on the Manhattan Project –

America’s secret development of the atomic bomb. Offered as a coating to metal parts,

Teflon™ PTFE (or simply Teflon™), has been widely available since then perhaps

most universally known as a non-stick coating for cookware.

PTFE is produced from the polymerization of TFE monomers (Fig. 2). This process

begins with the creation of TFE from hydrofluoric acid (HF), calcium fluoride

(mineral, aka fluorspar or fluorite, CaF2), and chloroform (CHCl3). These reactants are

heated to temperatures in excess of 600 °C (1112 °F) to produce TFE gas which is then

cooled and purified as a liquid. From this step, TFE monomers are polymerized

typically via a radical initiator (such as peroxide, R–O–O–R) to create PTFE. Varying

processing protocols can be used to produce PTFE as pellets, powder, or dispersion in

aqueous solution [2].

Figure 2: PTFE (polytetrafluoroethylene) and its immediate

synthetic precursors. PTFE is made from the polymerization of

tetrafluoroethylene (TFE) monomers via a radical reaction.

PTFE is routinely viewed as the premier fluoropolymer plastic because of its many

superior beneficial properties over other fluorine-containing polymers. PTFE’s

properties are the result of it containing exclusively carbon-fluorine (C–F) bonds along

its carbon-carbon backbone (C–C) (Fig. 3). C–F bonds are very strong, short, and

highly unreactive. As such, the C–C backbone of PTFE is surrounded by a sheath of

unreactive fluorine atoms. This unique structure is responsible for many of the

properties for which PTFE is known. PTFE’s very low coefficient of friction

(“slippery” feel), low water absorption, and chemical resistance are all the result of

PTFE’s saturated C–F bonding.


NEW FOCUS ON PTFE

A

B

C

Figure 3: Structure of PTFE. A) Stereo bond-line drawing, B) ball-and-stick

model, and C) space-filling model of PTFE (shown as zig-zag depiction).

(Conformations of PTFE are variable during melt and amorphous phases).

PROCESSING

PTFE is not melt-processable because of its very high melt viscosity. Instead,

techniques such as paste extrusion whereby the PTFE resin (as powder) is blended with

a lubricant (typically a hydrocarbon) may be used (Table 1). This method is used to

create a preform which can then be used to produce items such as tubing, sheets, and

tapes. PTFE products can also be fabricated by sintering and molding into a billet. This

PTFE material can be heated and ram-extruded to create rods, tubing, or pipes.

Furthermore, primary PTFE products such as rods can be calendered to produce sheets

and membranes. Compression molding can also be used with PTFE for molded parts.

Thus, despite its limitation of not being melt-processable, PTFE is nevertheless widely

commercialized for a myriad of industries and environments.

Processing Method Suitability

Injection molding No

Extrusion

(profiles, films, sheet, tubing, heat

shrink tubing, and cable coating)

Yes (as paste)

Blow molding No

Compression molding

(preform and sintering) Yes

Impregnation and coating Yes

(coating, as powder)

Table 1: PTFE processing suitability. Although

PTFE is not melt-processable, it is amenable to several

other production methods including paste extrusion and

compression molding.


NEW FOCUS ON PTFE

PTFE PROPERTIES

PHYSICAL AND MECHANICAL

PTFE is often considered the “gold standard” of fluoropolymers due to many of its

exceptional properties. Indeed, because of these attributes, the later development of

other polymers such as FEP and PFA were the attempts to refashion the properties of

PTFE but in a melt-processable form. PTFE is the most saturated straight-chain

perfluorinated polymer, and this quality is responsible for nearly all of its properties

(Tables 2 and 3). PTFE’s C–C bonding and short, strong, and polar C–F bonding

create a rigid molecule resulting in a material with high crystallinity and density.

Likewise, the sheath of fluorine atoms surrounding the C–C backbone of PTFE is

highly hydrophobic and unreactive. These latter qualities in particular render PTFE

chemically inert in the body and give it biocompatibility as a Class VI medical grade

polymer. PTFE is otherwise highly resistant to a very broad range of chemicals. Despite

some limitations in mechanical properties, PTFE surpasses other fluoropolymers in

almost all beneficial attributes.

Property ATSM Value

(natural polymer)

Appearance – Translucent

Density (g/cm3) D792 2.17

Specific Gravity D792 2.16

Water Absorption (50% rh; %) D570/ISO

62-1 < 0.01

Refraction Index D542 1.35

Limiting Oxygen Index (LOI) D2863 95

Biocompatible *USP

Class VI Yes

Chemical Resistance – Excellent

Sterilization – ETO, autoclave

Table 2: PTFE typical physical properties. PTFE’s physical

properties place it in a small group of fluoropolymers whose

traits are superior to almost all other polymer plastics.

(Methods are ASTM test standards except where indicated

by *).


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PTFE’s mechanical properties are closely tied to its crystallinity. PTFE’s highly

homogenous linear structure results in a very high degree of crystallinity, exceeding

90% for newly produced material. (Compare this to PVDF, another highly

homogenous fluoropolymer, whose crystallinity generally falls in the range of 40-

70%). Highly crystalline materials typically exhibit brittleness and comparatively

reduced tensile strength and elastic properties. To combat these seeming shortfalls,

PTFE can be produced with a variety of fillers to address specific mechanical (or other)

properties. Additionally, processing details can be varied to influence crystallinity. On

the other hand, a high crystalline nature leads to increased hardness and impact

strength, critical considerations for applications involving wear. Similarly, PTFE’s

extraordinarily smooth surface significantly mitigates wear for parts and other surfaces

that come into contact with PTFE components. Lastly, also due in large part to its

crystalline nature, PTFE shows excellent temperature stability leading to consistent

mechanical properties at high and low temperatures.

Property ASTM Value

(natural polymer)

Tensile Strength (MPa) D638 20 – 35

Elongation at Break (%) D638 200 – 550

Modulus Of Elasticity (GPa) D638 0.39 – 0.60

Flexural Modulus (GPa) D790 0.49 – 0.59

Flexural Strength (GPa) D790 No break

Hardness (Shore D) D2240 50 – 65

Impact Strength (23 °C; J/m) D256 186

Coefficient of Friction D1894 0.02 – 0.20

Table 3: Typical PTFE mechanical properties. While

comparatively mechanically weak, PTFE exhibits very low

coefficient of friction and very high hardness.

THERMAL

PTFE’s strong C–C and C–F bonding are once again demonstrated in its thermal

properties (Table 4). The C–F bonds of PTFE require more energy (116 kcal/mol) to

break than even C–H bonds (99 kcal/mol) [3]. This gives PTFE a very high operating

temperature – up to 260 °C (500 °F) – among the highest of any fluoropolymer. PTFE

is extremely difficult to burn requiring at least 95% oxygen concentration. (By

comparison, normal air contains only about 21% oxygen). These flammability traits of

PTFE are especially beneficial for PTFE components used in sensitive or critical

applications such as aerospace and automotive environments. Of note, however, is that

PTFE undergoes a slight decrease in volume (~ -1.8%) from 30 °C to 19 °C (86 °F to


NEW FOCUS ON PTFE

66 °F) as it shifts to a more tightly wound canonical PTFE helix [4, 5]. This volume

reduction can be an important consideration for PTFE parts that require fine tolerances.

Below its helical conformation temperature (19 °C; 66 °F), PTFE nonetheless exhibits

excellent functional behavior down to -200 °C (-328 °F). PTFE retains many of its

most important performance attributes over a temperature range that is among the

broadest of any fluoropolymer.

Property Method Value

(natural polymer)

Thermal Conductivity (W/m-K)

D433 /

ISO 22007-4 /

C-177

0.17 – 0.30

Maximum Service Temperature (°C) UL 746 260

Minimum Service Temperature (°C) UL 746 -268

Melting Point (°C)

D4591 / D3418/

ISO 12086 /

DOW Method 327

Decomposition Temperature (°C) E1131 505

Coefficient of Thermal Expansion,

linear (μm/m-°C) D696 100

Flammability Rating (UL 94) D2863 V-0

Table 4: PTFE thermal properties. PTFE exhibits the broadest temperature

usage range of any fluoropolymer. PTFE is also highly resistant to burning

and requires very high oxygen content to combust. (Methods are ASTM test

standards except where indicated).

ELECTRICAL

PTFE stands out among polymer materials for its electrical properties (Table 5).

PTFE’s highly polar C–F bonds support superior dielectric aspects and across a very

broad frequency span. Indeed, PTFE’s dissipation factor and dielectric constant are

very stable from room temperatures down to temperatures as low as -250 °C (-418 °F)

and at frequencies up to 10 GHz [6, 7]. PTFE dielectric breakdown (aka breakdown

voltage) can be improved further still by minimizing voids within the PTFE material

during the manufacturing process [6]. PTFE’s dielectric strength likewise is highly

insensitive to heat and thermal aging [7]. On whole, PTFE’s insulating properties

outperform almost every other solid material.


NEW FOCUS ON PTFE

Property ASTM Value

(natural polymer)

Dielectric Constant (1 MHz) D150 2.1

Dielectric Strength (V/mil) D149 /

IEC 60243-1 457 – 483

Volume Resistivity (Ω–cm) D257 /

IEC 60096 < 1018

Table 5: PTFE electrical properties. PTFE’s perfluorinated

nature containing strongly polar C–F bonds results in highly

beneficial insulating characteristics. PTFE’s dielectric benefits

remain almost unchanged across a very broad frequency range.

(Methods are ASTM except where indicated).

FINISHING

PTFE is amenable to most common machining and fabrication techniques. A variety

of products, shapes, and types can be produced from solid processed PTFE. Basic

machining techniques such as turning, threading, tapping, grinding, skiving, drilling,

and others are applicable to PTFE with no machine modifications. Despite being a

plastic, however, PTFE exerts tool wear comparable to stainless steel [7]. Conversely,

this aspect allows PTFE parts to be turned to very fine tolerances < ± 0.001″

(0.025 mm) [7]. A brief heat treatment (annealing) is recommended to attain tightly

controlled tolerances [7]. PTFE can also be pre-expanded to produce PTFE heat shrink

tubing for encapsulation uses; this specialty product hardens or sets in a solid-to-nearly-

solid form. A variation on sheet and membrane PTFE is that these forms can be

stretched or mechanically expanded (differently than for PTFE heat shrink) in a highly

controlled manner to create expanded PTFE (ePTFE). This material is microporous

and has opened up many new avenues for PTFE use, particularly in the medical sector

and for specialized filtration applications. Finished PTFE goods can be bonded but

most often will require a chemical etching treatment using concentrated alkali metal

hydroxide solutions or metal hydrides [6]. PTFE can be colored though pigment

options are limited to those that tolerate the high PTFE processing temperatures.

PTFE’s post-extrusion-friendly traits have played a significant role in expanding its

preference and high-performance uses into a broad range of industries.


NEW FOCUS ON PTFE

APPLICATIONS

PTFE continues to occupy the largest market share of commercial polymer plastics

today [2]. Its unreactive, temperature insensitive, and highly crystalline nature have

resulted in PTFE products in widespread use across almost all commercial industries

(Table 6). From pharmaceutical to aerospace and automotive to chemical and electrical

industries, PTFE is seated in a unique place where it can fill these and many more

niches. Extruded PTFE can now be formed into such products as tubing, extruded over

wire, monofilament with and without profiles, compression molded, calendered into

membranes and sheets, and stretched to create heat shrink and microporous ePTFE.

This high-performance fluoropolymer can also be extruded with special profiles which

would otherwise be difficult or impossible to produce (Fig. 4). Raw PTFE parts can be

machined and modified to form highly specific finished goods as well as being suited

for the creation of products that have yet to be conceived. These manufacturing and

material traits of PTFE have led to its dominance in the fluoropolymer industry.

Application or Industry Key Benefits

Fluid handling

tubing, piping

joint seals

vessel linings

Chemical resistance

ePTFE Filtration

Automotive

Aerospace

Chemical resistance

Dielectrics

Temperature resistance

Medical Biocompatibility, chemical resistance

catheter mandrel

catheter base liner

ePTFE (as implantables)

lubricity

bondable (etched)

microporous, cellular ingrowth

Electrical Dielectrics, insulation

(as coating over wire)

Heat shrink Encapsulation, protection

(AMS-DTL-23053™/12 compliant)

Fiber Optics Lubricity

Abrasion resistance

Table 6: Survey of PTFE applications. PTFE’s temperature tolerance,

ability to be sterilized, and ability to be produced in forms such as ePTFE and

PTFE heat shrink have made it one of the most widely used fluoropolymers.


NEW FOCUS ON PTFE

Figure 4: Extruded PTFE profiles. Zeus can extrude PTFE

in nearly unlimited customized profiles (and multilumen

forms) like those shown here which otherwise would be

difficult to produce.

SUMMARY

PTFE was the first in a long line of fluoropolymers that has become among the most

important commercial plastics ever created. This durable homopolymer, composed

entirely of very stable and unreactive C–F bonds along its carbon backbone, is the most

fluorinated of the fluoropolymers. This characteristic allows PTFE to exceed nearly all

other fluoropolymers in critical yet beneficial attributes and across the most diverse

conditions. PTFE’s strong bonding elevates its melt temperature and is partly

responsible for its high melt viscosity. As a consequence, PTFE cannot be melt-

processed but instead can be paste and ram extruded. PTFE parts can also be formed

via compression molding. PTFE can be machined using conventional techniques and

equipment and can even be stretched to form heat shrinks and microporous ePTFE. For

bonding applications, PTFE can be chemically etched to overcome its extremely non-

stick surface. This product versatility has contributed to PTFE becoming the most

widely used industrial plastic.

PTFE specifically stands out with respect to its properties. As a fully fluorinated

polymer (apart from its C–C backbone), PTFE is the archetype for the fluoropolymers

that followed it. PTFE’s C–F and C–C bonding convey extremely broad chemical

resistance to the material. PTFE’s strong bonding gives it superior temperature

stability, and PTFE is highly disinclined to burn requiring > 95% oxygen to sustain

combustion. PTFE’s superb insulating characteristics are highly prized in electrical

applications. With some mechanical limits, PTFE possesses very high hardness along

with concomitant very low coefficient of friction. These two attributes are especially


NEW FOCUS ON PTFE

beneficial for part wearing. Viewed collectively, PTFE’s beneficial properties eclipse

those of almost every other polymer (Table 7).

Advantages / Benefits (+) Limitations (–)

Broad-spectrum chemical resistance

High temperature tolerance

(260 °C / 500 °F)

Low temperature tolerance

(<-200 °C / -328 °F)

Low coefficient of friction

Superior weathering

Poor mechanical strength

Not melt-processable

Chemical etching needed for

adhesive and bonding

Limited radiation resistance

High comparative cost

Table 7: PTFE advantages and limitations. PTFE can be used in

almost every application where mechanical strength is not required with

the exception of radiological uses. Broad chemical, temperature, and

weathering resistance place PTFE in a group by itself in the landscape

of performance fluoropolymer extrusions.

ABOUT ZEUS

Zeus is the world’s leader in polymer extrusion technologies. For over 50 years, Zeus

has been serving the medical, aerospace, energy exploration, automotive, and fiber

optics industries. Headquartered in Orangeburg, South Carolina, Zeus employs

approximately 1,500 people worldwide and operates multiple facilities in North

America and internationally. You can find us at www.zeusinc.com.

CONTACT US

More information regarding the material discussed here is available by contacting one

of Zeus’ technical account managers. They can be reached in the US toll-free at

1-800-526-3842 or internationally at +1-803-268-9500. You can also email us at

[email protected].

Zeus Industrial Products, Inc.

3737 Industrial Blvd.

Orangeburg, SC 29118

USA

Zeus Ireland - Tel +353-(0)74-9109700

Zeus China - 电话 +(86)20-38791260

http://www.zeusinc.com/

mailto:[email protected]


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RESINATE SPECIAL EDITIONS

As part of our efforts to provide you with relevant, useful, and timely information, we

regularly review our content available through our multiple outlets. Our review also

includes previous publications which we have retained in our library. RESINATE

Special Editions are our updated and revised versions of some of these previously

issued yet valuable learning tools we wish to continue to provide to our customers.

REFERENCES:

1. Plunkett RJ: The History of Polytetrafluoroethylene: Discovery and Development.

In: High Performance Polymers: Their Origin and Development: 1986// 1986;

Dordrecht. Springer Netherlands: 261-266.

2. Gardiner J: Fluoropolymers: Origin, Production, and Industrial and Commercial

Applications. Australian Journal of Chemistry 2015, 68(1):13-22.

3. Carruth G, Ehrlich E: Bond Energies. In. Edited by Carruth G, vol. 1, Library edn.

Tennessee: Southwestern; 2002.

4. McCrum NG: An internal friction study of polytetrafluoroethylene. Journal of

Polymer Science 1959, 34(127):355-369.

5. Weir CE: Transitions and phases of polytetrafluoroethylene (teflon), vol. 50; 1953.

6. Ebnesajjad S: 4 - Introduction to Fluoropolymers A2 - Kutz, Myer. In: Applied

Plastics Engineering Handbook. Oxford: William Andrew Publishing; 2011: 49-60.

7. DuPont: PTFE Handbook. In.; 1996.

new focus on ptfe - polymer extrusion & tubing …...teflon ptfe (or simply teflon ), has been...

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